Self-regulating heating cables have an inherent useful lifetime, due to gradual changes in their materials as a result of exposure to environmental temperatures. Over time, the power output of heating cables diminishes, until the heating cables no longer efficiently heat their surroundings as intended and must be replaced. The diminishing of the power output of the heating cable relative to the initial power output is referred to as aging, and is a major concern in industry. All lifetime exposure, whether generated internally by the heater itself or received externally through the environmental conditions at the deployment site, has a cumulative aging effect on the heater. These environmental conditions include, but are not limited to, thermal oxidation of the core polymer material due to normal operation at an ambient temperature, degradation of the electrical contacts, mechanical damage or excessive heating beyond rated specifications which can lead to partial melting of the polymer material.
The most important aging mechanism is thermal oxidation of the core polymer material, which is an inherent property of self-regulating heaters operated under any conditions. This thermal oxidation mechanism will herein be described as “thermal aging”. Thermal aging might be expressed through the percentage of the remaining power output of the heater relative to the original power output. The power output due to this mechanism versus time and exposure temperature is described by an Arrhenius-law equation:Power=Power0*exp{(A−B*time*exp(−C/Temperature))},where A, B and C are material constants specific to a certain type of heating cable, “Temperature” refers to the temperature in degrees Kelvin of the heating cable, “time” is the exposure time to that temperature, and Power and Power0, respectively, refer to the power output in Watts of the heater after the exposure time and at initial installation. However, in many real applications, heaters are not exposed to constant temperatures at all times. Rather, the temperature can fluctuate to include very high temperatures for limited times, particularly in industrial applications. Because the exposure temperatures over time may be unknown, it is usually not possible to accurately predict the thermal aging of a heater in a certain application. It is therefore important to track the thermal age of a heating cable while installed. Thermal age mustn't be confused with the actual or installed age of the heater, but contains the integrated temperature history to which it was exposed over time. The reason for the diminished power output is an increase in the cable's resistivity that results from thermal aging. Hence, in principle, measurements of resistivity or power could serve as an indicator of thermal aging. However, the measurement loses its value as an indicator if the sample is cut, spliced, damaged or otherwise changed, since such measurements are referenced to an initial baseline measurement taken. More importantly, in order to be able to use simple power or resistivity measurements conducted under deployment conditions to determine the thermal age of the heating cable, the complete temperature profile along the cable installation would have to be known, which would defeat the purpose of installing a self-regulating heating cable, which self-regulates its power output to the temperature profile. Further, a resistivity measurement is only meaningful if the heater power output has reached thermal equilibrium; resistivity measured shortly after powering up the heater is confounded by an inrush effect.
In order for customers at deployment sites to better plan their schedule for heating cable replacement, an improved material-characteristic-based thermal age indicator is needed which does not require specific knowledge of the history of a particular deployed heating cable. The indicator should be inexpensive and non-invasive. In particular, it would be advantageous to utilize in situ measurements of heating cable characteristics, such as voltage and current, that may already be monitored by a controller for other purposes.